1. Field of the Invention
This invention relates to a structure for supporting a sound board of an upright piano.
2. Prior Art
In the piano, a hammer strikes a string to produce vibration energy, which is subjected to diffusing propagation onto an overall area of a sound board. Thus, the piano is capable of producing a sound whose tone volume is sufficiently large. The piano provides a structure for supporting a sound board. According to such a support structure of the sound board, in the case of an upright piano, a peripheral portion of the sound board is securely fixed to a sound board bearing shim (or sound board base) (which may be called "uchimawashi"in Japanese), which is then fixed to a back frame.
The support structure of the sound board of the upright piano conventionally known is shown in FIGS. 4A and 4B. Herein, FIG. 4A is a front view of the support structure while FIG. 4B is an enlarged sectional view taken along the line I--I of FIG. 4A. A numeral `1` designates a number of posts, which correspond to a back frame having a vertical lattice pattern. The back frame 1 is constructed by an upper beam 2, a middle beam 3 and a bottom sill 4, for example. The back frame 1 is stored in a perpendicular form within a body of a musical instrument (not shown). A pin block 5 is fixed to an upper end portion of a front side of the back frame 1 in connection with the upper beam 2 and the middle beam 3. In addition, a sound board bearing shim 7 is securely fixed to the back frame 1. An upper end portion of a frame 8 which is formed by casting is fixed to the back frame 1 and the pin block 5. The sound board bearing shim 7 is constructed by a framework 11 and two diagonal beams 13a, 13b. Herein, the framework 11 is constructed by assembling an upper frame beam 9a and a lower frame beam 9b as well as vertical frame beams 10a, 10b. Each of the above beams is made by a hard rectangular lumber (i.e., hard beam) whose size in section ranges from 30 mm to 40 mm, for example. The diagonal beams 13a, 13b are arranged inside of the framework 11. That is, the diagonal beam 13a is arranged to form a small triangle area at an upper-left corner of the framework 11. Similarly, the diagonal beam 13b is arranged to form a small triangle area at a lower-right corner of the framework 11. A peripheral portion of a sound board 6 is attached (or glued) to faces of the above beams by an adhesive agent. An upper portion (or upper frame) of the sound board bearing shim 7 is glued to the middle beam 3, whilst a lower end portion is glued to the bottom sill 4.
A crown is formed for the sound board 6 to have a convex curved surface at its front-face side. A base bridge 18 and a treble bridge 19 are respectively glued to a front face of the sound board 6. Herein, the base bridge 18 supports intermediate portions of strings 15 provided for a bass section, whilst the treble bridge 19 supports intermediate portions of strings 16, 17 provided for a tenor section and a treble section respectively. Tuning pins 20 are planted onto an upper end portion of the frame 8, whilst frame pins 21 are planted onto a lower end portion of the frame 8. Ends of the strings 15, 16 and 17 are terminated by the tuning pins 20 and the frame pins 21. Thus, a certain level of tension is imparted to the strings. A number of sound board ribs 22 are glued to a back surface of the sound board 6. Herein, the sound board ribs 22 are arranged substantially perpendicular to a grain direction of the sound board 6. Both ends of the sound board ribs 22 are glued to the sound board bearing shim 7.
In the conventional support structure of the sound board described above, vibration of the sound board 6 is transmitted to the back frame 1 through the sound board bearing shim 7. Hence, if a high impedance (or high rigidity) is set to the post of the sound board bearing shim, a sound board assembly as a whole would have a high impedance to string vibration. The impedance of the sound board of the conventional upright piano is so high that it is difficult to optimize the impedance matching between the strings and sound board; therefore, the conventional upright piano suffers from a problem that reduction occurs in sound quality and tone volume.